The present invention relates to a super finishing stone which performs super finishing work on a raceway surface of an object to be worked such as a power transmission disk of a toroidal continuously variable transmission and rings of a thrust ball bearing, which has a raceway surface for receiving a thrust load, and a super finishing method.
In the disk of the toroidal continuously variable transmission, for example, the raceway surface having an arc-shape in cross section is continuously formed in a circumferential direction (Reference should be made to Japanese Patent Publication No. JP-A-2002-295618, for example). FIG. 5A is a sectional view showing a disk 10 of this type and FIG. 5B is a front view thereof. In the drawings, the disk 10 is formed with a fitting hole 10b to be fitted to a shaft (not shown) at a center thereof, and a raceway surface 10a on one surface thereof. The raceway surface 10a which has been formed in a predetermined shape in advance, as shown in the drawings, is further super finished by a super finishing stone, by rotating the disk 10 around its axis.
FIG. 6 is a sectional view of the disk 10 in a state where a super finishing stone (grindstone) 11 is abutted against the raceway surface 10a. The super finishing stone 11 is supported by a support part 12, and oscillates around a center P of curvature of the raceway surface 10a within a predetermined angle range. With this super finishing stone, the super finishing work is performed on the raceway surface 10a. 
In FIGS. 5A and 5B, seven sections from section A-A to section G-G in the drawing as desired sections which intersect a radial direction of the disk 10 are imagined, for example. The raceway surface 10a which is a three-dimensional curved surface has a flat shape at an intermediate section D-D, and comes into a convex shape which is gradually swelled toward a center of the section in order from the section E-E, the section F-F, to the section G-G inwardly in the radial direction. On the other hand, the raceway surface 10a comes into a concave shape which is gradually recessed toward a center of the section in order from the section C-C, the section B-B, to the section A-A.
FIGS. 7A and 7B are respectively a front view and a side view of the super finishing stone 11 which has been conventionally used. The super finishing stone 11 has a constant width W, and a so-called hog-backed shape as seen in a side view. Seven sections from section A-A to section G-G corresponding to the sections of the raceway surface 10a are imagined on an abrasive (grinding) surface 11a after the work (disk) has been oriented. The abrasive surface 11a has a flat shape at an intermediate section D-D, and comes into a concave shape which is gradually recessed toward a center of the section in order from the section E-E, the section F-F, to the section G-G inwardly in the radial direction. On the other hand, the abrasive surface 11a comes into a convex shape which is gradually swelled toward a center of the section in order from the section C-C, the section B-B, to the section A-A. In short, surface profiles of the raceway surface 10a and the abrasive surface 11a match with each other in a convex-concave relation.
The aforesaid super finishing stone 11 can oscillate as described above. In case where the super finishing stone 11 has oscillated outward in the radial direction by an angle d ω from a position where the surface profiles of the raceway surface 10a and the abrasive surface 11a match with each other (for example, a center of an angle of oscillation), as shown in FIG. 6, the abrasive surface to be abutted against the raceway surface 10a is displaced. For example, in the above described case, the section E-E of the abrasive surface is abutted against the section D-D of the raceway surface. FIG. 8 is a view showing a state of contact between the raceway surface 10a and the abrasive surface 1a in case where they are displaced from each other. In FIG. 8, solid lines represent the profile of the abrasive surface 11a, and dotted lines represent the profile of the raceway surface 10a. Curvature becomes larger in order from a convex 1, a convex 2, to a convex 3, and in order from a concave 1, a concave 2, to a concave 3.
In this case, an entirety of the abrasive surface 11a as seen at one section will not come into contact with the raceway surface 10a, but only right and left edges of the abrasive surface 11a in a sectional shape come into contact with the raceway surface 10a. In fact, however, regions close to the edges also come into contact, as the curvature becomes smaller. However, as shown in FIG. 8, the contacted regions are concentrated to opposite side ends as going inward in the radial direction, which leads to high edge load condition. As the results, there is a problem that working conditions become inconstant depending on positions, and unevenness in super finishing may occur.